Coplanar Waveguide (CPW) Loaded With an Electromagnetic Bandgap (EBG) Structure: Modeling and Application to Displacement Sensor

In this paper, a microwave displacement sensor that is based on electromagnetic bandgap structure is proposed. The periodic air holes realized in a silicon substrate cause bandgap in the transmission response of coplanar waveguide (CPW) line printed on this substrate. The proposed sensor consists of a CPW line loaded by periodic air holes along the transmission line and another periodic substrate that is located on the top of this CPW line. If the upper substrate is moved along the line, the maximum insertion loss will change, and therefore, a displacement sensor based on notch in the transmission coefficient is realized. The distance dynamic range of this sensor is limited by half of the period of the air holes. Moreover, the sensitivity of the sensor for a specific period depends on the number of unit cells. The sensitivity is
_~
4 dB/100 μm with 2.5-mm linear dynamic range (corresponding to 62-dB variation in maximum insertion loss) for ten unit cells with a period of 5 mm. The response of the sensor is almost independent of temperature, and the sensor needs a simple fixed frequency circuit to measure the displacement. The transmission line model of the sensor is also presented and discussed.